1. Field of the Invention
The present invention relates to cosmetic compositions that have enhanced rheological properties. More particularly, the present invention relates to sunscreen compositions that have enhanced rheological properties before and after application.
2. Description of the Prior Art
Cosmetic compositions, including sunscreens, are commercially available in various forms, such as lotions, creams, gels, and the like and are well known in the art. These compositions are typically formulated to have optimal rheological properties at or close to room temperature. One major drawback with these compositions is that they degrade rheologically at elevated temperatures, especially above room temperature. At these elevated temperatures, it has been found that the compositions no longer maintain the desired viscosity, resulting in a product that undesirably may drip or run during dispensing and/or application.
While all cosmetic compositions may be exposed to elevated temperatures, sunscreen compositions are especially susceptible to exposure to elevated temperatures due to the nature of their use. Sunscreen compositions are applied to the skin to protect the skin from the sun's ultraviolet rays that can lead to erythema, a reddening of the skin also known as sunburn. Sunlight or ultraviolet radiation in the UV-B range has a wavelength of 290 nm to 320 nm and is known to be the primary cause of sunburn. Ultraviolet rays at a wavelength of 320 nm to 400 nm, known as UV-A radiation, produces tanning of the skin. However, in the process of doing so, the UV-A rays can damage or harm the skin.
Besides the immediate malady of sunburn, excessive sunlight exposure can lead to skin disorders. For instance, prolonged and constant exposure to the sun may lead to actinic keratoses and carcinomas. Another long-term effect is premature aging of the skin. This condition is characterized by skin that is wrinkled, cracked and has lost its elasticity.
As stated above, sunscreens are typically formulated with the goal of inhibiting skin damage from the sun's rays. The sunscreen composition filters or blocks the harmful UV-A and UV-B rays that can damage and harm the skin. It is believed that sunscreen agents accomplish this by absorbing the UV-A and/or UV-B rays.
Consumers consider many factors when purchasing a sunscreen product, such as, the sun protection factor (SPF), how durable the product is after applying it over the skin, the shelf life of the product, and product form (i.e., lotions, gels, creams, and sprays). Another important and influential property of a sunscreen product considered by a consumer is how the product dispenses and how well the product spreads over the skin. Typically, consumers want a sunscreen that does not drip and/or run from the dispenser orifice, from the hand during application, or once applied to the skin. However, when exposed to the sun or elevated temperatures, typically sunscreen viscosities significantly decrease. This causes the sunscreen to drip and run from both the orifice of the dispenser and on the skin, making it very difficult for a user to evenly apply the sunscreen to the skin. In addition, the viscosity breakdown may result in a loss of product aesthetics, thus the product does not have a smooth, silky feel when applied to the skin, which is another important factor considered by consumers.
The cosmetic compositions of the present invention overcome the prior art deficiencies by providing a composition with a stable viscosity over a broad temperature range. As a result, the composition does not drip or run when dispensed and/or applied to the skin. In addition, the composition imparts an enhanced soft, silky feel when applied.